A number of toners having on the surface thereof a fluorine-containing compound have been proposed for obtaining improved developing characteristics. For example, JP-A-58-66950 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a capsule toner having an organic fluorine compound on the surface of capsules; JP-A-59-181358 discloses a capsule toner having a fluorine resin on the surface of capsules; JP-A-60-126657 discloses a capsule toner whose outer shell is formed by interfacial polymerization of a fluorine-containing compound; JP-A-61-120160 discloses a technique of imparting chargeability to toner particles which comprises reacting a fluorine-containing alcohol with a carboxyl group or an isocyanate group present on the outer shell of the particles; and JP-A-63-177145, JP-A-63-177147 and JP-A-63-177148 disclose a capsule toner having formed on the outer shell thereof a high-molecular chain by graft polymerization of a compound having a fluorine atom as a charge control group.
However, none of these toners proposed to date is sufficiently satisfactory due to the respective disadvantage described below.
The toner disclosed in JP-A-58-66950 has dependence of chargeability on the environmental condition, particularly humidity because the organic fluorine compound present on the capsule surface contains not only a fluorine-containing hydrophobic group but a hydrophilic group.
The fluorine resin disclosed in JP-A-59-181358 is a homo- or copolymer having a structure like a hydrocarbon with its hydrogen atom displaced with a fluorine atom and therefore has poor adhesion to the outer shell and easily releases therefrom.
When the technique of JP-A-60-126657 is followed, one encounters with difficulty in obtaining both mechanical properties of the outer shell and chargeability of the toner as a whole from the same outer shell-forming material. In other words, there is involved the problem of narrow choice of materials in the production.
According to the invention disclosed in JP-A-61-120160 where an outer shell is formed by interfacial polymerization, since either a carboxyl group or an isocyanate group is a functional group participating in interfacial polymerization, it is difficult to make such a group present on the surface of the outer shell. Further, since the reaction between this functional group and an alcohol does not sufficiently proceed in an aqueous solution, there is no means but to use an organic solvent. As a result, an apparatus for solvent recovery is required, and the equipment should be proof against explosion, which incur cost.
In JP-A-63-177145, JP-A-63-177147, and JP-A-63-177148, vinyl fluoride is the only example given as a grafting monomer containing a fluorine atom. Moreover, vinyl fluoride is gaseous at room temperature and requires a special reaction apparatus to be grafted to the capsule surface.
Apart from the use of a fluorine-containing compound, it has been proposed to maintain charging stability of a toner by using a metal complex salt, etc. However, it is difficult to control dispersibility of a metal complex salt in a toner composition. Besides, most of metal complex salts are inapplicable to color toners because of their own color.
Having studied on negative chargeability of microcapsule toners, the inventors previously proposed to adhere a polymer comprising a specific fluorine-containing vinyl monomer to the capsule surface (JP-A-3-220561). However, the capsule toner having a fluorine-containing polymer on the surface thereof as disclosed in JP-A-3-220561 proved to undergo deterioration in electrophotographic image quality. This is because core-forming fine particles of low electrical resistance, such as a magnetic powder, carbon black and a pigment, are completely covered with an insulating capsule wall and the fluorine-containing polymer is also insulating so that charge exchange among capsule toner particles is hindered, resulting in broadening of charge distribution.
In addition, the conventional capsule toners have insufficient cleaning properties and they tend to remain unremoved from a photoreceptor under a low temperature and low humidity condition. In general, a toner having a capsule structure is required to have improved cleaning properties so as to clean a photoreceptor without imposing a large stress on the toner particles. In this connection, the conventional capsule toners were insufficient.